The present invention relates to an electrically insulating film, particularly an insulating film to be formed on a surface of a conductor substrate, and a method for preparing the same.
Hitherto, the surfaces of conductors have been treated by applying a tape made of a resin on them, or coating them with a resin layer, in order to secure an insulating property of the surfaces of conductors. When a thin insulating film is required, for instance, it is produced by applying a precursor of a resin on the surface of a conductor and baking it with heat as represented by varnish treatment for enamel wires.
With the recent miniaturization of portable electronic appliances, designs of electronic components and motors are directed to their smaller and thinner sizes, and their smaller power consumptions commensurate with the miniaturization of electronic appliances, and thus thinner insulating films are now being required.
However, the insulating film obtained by the above-mentioned surface treatment with a resin tape generally has an insufficient thickness of only not less than dozens of .mu.m due to restricted durability and workability of the resin tape. On the other hand, varnish-coated films for enamel wire can be reduced in thickness, but, when the thickness is reduced to some .mu.or less, they easily develop pinholes and lose their insulating property. Similarly, if treated by the method of high speed winding as is often used for production of motor coils, the insulating film loses its insulating property due to possible extension and bending of enamel wire during the winding process, and its withstand voltage per .mu.drops to about 500 V.
The above phenomenon may be explained that the resin constituting the insulating film is bonded to the surface of metal only mechanically and is easily separated or transferred by an external force.